The present invention relates to an implant tool for implanting a tissue-stimulating lead having an extendable/retractable positive fixation tip, such as an endocardial lead having an extendable/retractable screw-in (helix) tip.
A tissue-stimulating lead is used with a tissue-stimulating device, such as an implantable pacemaker, in order to electrically couple the stimulating device to a desired tissue location. When the stimulating device is a cardiac pacemaker, for example, the tissue-stimulating lead, also referred to as a "pacing lead," connects the pacemaker's electrical circuits directly with a desired chamber of the heart. Access to the heart is usually achieved transvenously, i.e., the lead is inserted into the heart through a major vein, such as the superior vena cava, thereby avoiding the trauma of open heart surgery. One or more electrodes at or near the end of the lead placed inside of the heart--referred to as the "distal end"--contact the cardiac tissue therein at the desired location. The electrode(s) are electrically connected via insulated conductors within the lead to an appropriate connector at the other end of the lead--referred to as the "proximal end." Herein, the terms "distal" and "proximal" are used to describe the ends of a lead, or tools or components or other elements used with a lead, that are respectively farthest or closest to a medical device to which the lead is, or will be, attached.
After an implantable lead is transvenously or otherwise implanted, and after various electrical tests have been made to confirm that the lead electrode(s) is positioned at the proper tissue location, the connector at the proximal end of the lead is detachably inserted into an appropriate mating connector of a medical device, such as a pacemaker, thereby electrically coupling the desired tissue location to the electrical circuits within the medical device.
The distal tip of the implantable lead is held at a desired tissue location by either positive fixation or passive fixation. Positive fixation (sometimes called "active fixation") involves the use of some type of mechanism or means for actively securing and holding the body tissue in contact with the distal tip. The most common type of positive fixation is achieved using a screw-in helix tip located at the distal end of the lead. Positive fixation is achieved by literally screwing the helix tip into the tissue. In contrast, passive fixation involves temporarily holding the distal tip near the desired location, e.g., through the use of tines near the distal electrode that become entangled with trabeculae located inside the heart, until some sort of tissue ingrowth can occur into the lead tip in order to firmly hold it in its desired position.
The advantages of positive fixation leads are well known in the art. A much more reliable tissue contact can be made and maintained using a positive fixation lead than can be made using a passive fixation lead. Such contact can be established immediately without having to wait for tissue ingrowth. Further, a positive fixation lead that has been properly affixed to body tissue is not easily dislodged in the event the patient is bumped, or suffers some other type of physical shock.
A common technique used to implant a positive fixation lead is to insert the lead transvenously into the desired tissue contact location, e.g., inside of the heart. However, such transvenous insertion requires that the positive fixation tip be maintained in a retracted position until the distal tip of the lead is at the desired tissue contact location. Otherwise, during the process of inserting the lead, the positive fixation tip, comprising, e.g., a sharp, protruding screw-in helix tip, could easily become entangled with and/or damage delicate body tissue at a location other than the desired tissue contact location. Thus, it is known in the art to use positive fixation leads having extendable/retractable helix tips. A simple construction of such a lead incorporates a sliding carrier that is inserted into a tip housing at the distal end of the lead. A screw-in helix tip is attached to the distal end of the carrier. Prior to placement of the lead, the carrier is axially pushed, from the distal end of the lead, to a retracted position such that the screw-in tip does not protrude from the distal end. Once the lead has been positioned for placement, the screw-in tip is extended by axially pushing the carrier, from the proximal end of the lead through the use of a stylet, to an extended position.
U.S. Pat. No. 4,649,938, issued to McArthur, shows a variation of this basic extendable/retractable construction that uses internal biasing means, e.g., a coiled spring, to maintain the carrier in its retracted position. Further, McArthur teaches the use of an O-ring to seal the carrier as it axially moves from its retracted position to its extended position. The O-ring rolls as the carrier moves, thereby facilitating low friction movement. An axial force must be applied to the stylet as the screw-in tip is screwed into body tissue. Once positive fixation has occurred, the internal biasing means tends to pull the tissue in contact with the electrode, thereby helping maintain a good tissue-electrode interface.
When a positive fixation tip is extended to its protruding position through the use of a stylet, it is necessary to apply and maintain an axial force to the stylet in order to keep the tip in its extended position. Otherwise, the body tissue contacting the tip tends to push the tip back into its retracted position. This axial force must continue to be applied as the positive fixation tip is affixed to the body tissue, e.g., realized by rotating the lead when the positive fixation tip is a screw-in helix tip. If too little axial force is applied, the tip may not remain fully extended, and may therefore not fully engage the body tissue. What is needed, therefore, is a convenient means of applying the correct axial force to a stylet in order to assure that the extendable/retractable tip is fully extended during the fixation process.
Even if the correct axial force is applied to the stylet in order to fully extend the positive fixation tip, this axial force must continue to be applied while the lead is rotated, or other action is undertaken to positively engage the tip with the body tissue. Heretofore, this has been at least a two-handed operation, applying the axial force to the stylet with one hand, while rotating the lead (or stylet, in some types of leads) with the other hand. Needless to say, such two-handed operation is cumbersome and difficult to do while maintaining the proper axial force on the stylet. There is thus a need in the art for an implant tool that facilitates the application of the proper axial force on the stylet while allowing the helix tip to be rotated the proper number of turns to assure proper fixation to body tissue. Preferably, such an implant tool would also be usable with one hand, thereby allowing the implant physician, or other medical personnel, to use his or her other hand for other important activities that must occur during the implant operation.